210 'carbon-fixation; 'assimilation 



were it not for the fact that the atmosphere is being continually 

 replenished with carbon dioxide which is produced in the 

 process of respiration carried on by all living things, and by the 

 combustion of coal, wood and other kinds of fuel containing 

 carbon. 



After entering into the cells of the leaf the carbon dioxide, 

 together with a certain proportion of water, undergo chemical 

 changes which result in the formation of soluble carbohydrates, 

 oxygen being also set free during the process. 



The carbon of the carbon dioxide thus becomes ' fixed,' and 

 a rapid accumulation of carbohydrates takes place in the tissues 

 of the plant, the oxygen escaping into the air. 



The process may be represented thus : — 



carbon dioxide + water = a carbohydrate + oxygen. 

 It has been customary among botanists to use the term 

 assimilation for the synthesis of carbohydrates by green plants in 

 this manner from carbon dioxide and water, but it would be 

 better to reserve the term for the conversion of foods into the 

 substance of the tissues, as is done by animal physiologists, and 

 employ another for this synthetical production of carbohydrates 

 which is peculiar to green plants. As the operation is dependent 

 upon light the term photosynthesis has been suggested and some 

 such term or the expression ' carbon-fixation ' is much to be 

 recommended instead of ' assimilation.' 



The exact nature of the carbohydrate first formed during the 

 process is not known. Von Baeyer suggested that formaldehyde 

 (CHjO) is first produced according to the equation — 



C02 + H20 = CH20 + 02, 

 and that this compound subsequently undergoes condensation 

 into a carbohydrate of the formula CgHjjOg. However, formal- 

 dehyde cannot be detected in the tissues in which the process 

 of ' carbon-fixation ' is going on, and although Bokorny's experi- 

 ments show that under certain conditions formaldehyde can be 

 utilised by plants for the production of carbohydrates, the view 



